SHEAR-LIKE MOUTH. 



279 



if prolonged, would meet in an acute angle above the palate. In 

 other words, the crowns lie not over, but alongside, one another, 

 so that the mouth resembles that of a flesh feeder (Figs. 279 and 

 280). The rows of teeth, therefore, do not grind, but cut, meeting 

 one another like the blades of a shears, in which the inner blade is 

 formed by the lower molars, the outer blade by the upper. When 

 confined to single teeth, this change most frequently affects the 

 fourth molars, because they are more liable to lateral displacements ; 

 but, as a rule, one whole row is affected, constituting simple shear 

 mouth ; occasionally both sides suffer (double shear mouth). 



The condition is brought about thus : The inner edge of the 

 lower molars and the outer edge of the upper are not worn away ; 



Fiu. 279. — Left-sided shear mouth 

 (from a photograph). 



Fig. 280.— Grinding surfaces 

 in shear mouth, in normal mouth. 



both continue to grow until the former meets the hard palate, while 

 the latter injures the gums of the lower jaw. Coupled with impaired 

 mastication, such wounds prevent the horse either taking or chewing 

 food. The overgown teeth sometimes penetrate the bone above, 

 and the hard palate may even be completely perforated. The side 

 thrust on the teeth loosens them, while wounds of the gum are apt 

 to produce inflammatory changes in the alveolar periosteum. Hence 

 " shear mouth," especially in old horses, generally produces alveolar 

 periostitis. 



Its causes are the same as those of angular or sharp mouth. A 

 narrow jaw and limited movement are notable predisponents. It 

 is unnecessary to discuss the correctness of Lorge's supposition that 

 the muscles of the jaw are imperfectly developed ; or Gunther's, 

 that the affection is due to variations in hardness of the molars, 



